Initial frequency synchronization mechanism

ABSTRACT

Method for acquiring frequency of a desired channel having a carrier frequency F MAIN , for a dynamic receiver frequency F MOBILE , from a starting frequency F START , in the presence of high power adjacent interfering channels, wherein the starting frequency F START  is shifted from F MAIN  by not more than a predetermined frequency gap ΔF, the method includes the steps of determining a first frequency boundary and a second frequency boundary, detecting channels within a filtering bandwidth, selecting a dominant channel from the detected channels, progressing the dynamic receiver frequency F MOBILE  towards the carrier frequency of the dominant channel, detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary, restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary, and repeating from the step of detecting channels.

This application is a continuation application of U.S. patent application Ser. No. 09/012,361, filed Jan. 23, 1998, now U.S. Pat. No. 6,175,722 issued Jan. 16, 2001, which is hereby incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to frequency acquisition in general and to frequency acquisition in the presence of high power adjacent channels, in particular.

BACKGROUND OF THE INVENTION

Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a initial frequency synchronization procedure, known in the art. The present example describes a closed loop automatic frequency control (AFC) mechanism.

FIG. 1B is a schematic illustration of frequency versus power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A.

Arrow 14 represents the frequency of a mobile unit which detects and attempts to lock and synchronize with the carrier frequency 10 of a base unit transmitter having a value of F_(BASE), which is located near by. In the present example the mobile unit further detects a carrier frequency 12 provided by a neighbor transmitter, having a value of F_(NEIGHBOR). The value of the mobile unit F⁰ _(MOBILE) is located between the values of the base unit frequency F_(BASE) and the neighbor mobile transmitter frequency F_(NEIGHBOR).

In the present example the mobile unit 14 detects the signals provided by base 10 and the neighbor 12 wherein the received power of the neighbor 12 is higher than the received power of the base unit 10.

According to conventional initial synchronization procedures, the mobile unit frequency is synchronized with the frequency having the highest received power, which in the present example is the neighbor frequency 12.

It will be noted that often the received frequencies are filtered so as to exclude undesired signals. Such a filter is represented by arc 16. These techniques often fail when the power of the undesired signal is significantly high.

Accordingly the synchronization mechanism of the mobile unit sets synchronization path towards the neighbor frequency F_(NEIGHBOR) and starts progressing its frequency 14 towards F_(NEIGHBOR). Finally the synchronization mechanism allows the frequency of the mobile unit 14 to acquire and synchronize with the frequency of the neighbor unit 12. This is shown in FIG. 1B by aligning line 12 and arrow 14. As can be seen, at this stage the frequency 10 of the base transmitter is filtered out by the filter 16.

A conventional synchronization mechanism provides frequency shifts within a limited range, determined by its structure, such as VCO voltage and the like. It will be appreciated by those skilled in the art that the F_(NEIGHBOR) can be located outside this range in such a case, F_(MOBILE), might get stuck at the boundary frequency value which is closest to F_(NEIGHBOR).

It will be appreciated by those skilled in the art that such situations, where the frequency of the mobile unit 14 is synchronized with the frequency of neighbor unit 12 instead of the frequency of the base unit 10, is not acceptable.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a novel method for performing accurate initial frequency acquisition in the presence of high power adjacent channels.

It is a further object of the present invention to provide a novel device for performing accurate initial frequency acquisition in the presence of high power adjacent channels.

In accordance with the present invention there is thus provided a method for acquiring frequency of a desired channel having a carrier frequency F_(MAIN), for a dynamic receiver frequency F_(MOBILE), from a starting frequency F_(START), in the presence of high power adjacent interfering channels.

The starting frequency F_(START) is shifted from F_(MAIN) by not more than a predetermined frequency gap ΔF. The method includes the steps of:

determining a first frequency boundary and a second frequency boundary;

detecting channels within a filtering bandwidth;

selecting a dominant channel from the detected channels;

progressing the dynamic receiver frequency F_(MOBILE) towards the carrier frequency of the dominant channel;

detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary;

restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary; and

repeating from the step of detecting channels.

According to another aspect of the present invention, one of the first frequency boundary and the second frequency boundary is F_(START)−ΔF, while the other is F_(START)+ΔF.

The method of the invention can also include the step of determining a frequency advance direction. The frequency advance direction can be fixed at the beginning of each frequency acquisition cycle, wherein the frequency acquisition cycle is determined from the point where F_(MOBILE) shifts from F_(START) until the point where F_(MOBILE) returns to F_(START).

The step of progressing can be performed in a frequency step F_(STEP). The value of the frequency step F_(STEP) can be infinitesimal with comparison to the predetermined frequency gap ΔF, or adjustable. Accordingly, the method can further include the step of adjusting the frequency step F_(STEP) after each step of detecting channels.

In accordance with another aspect of the present invention, there is provided a device for acquiring frequency of a desired channel having a carrier frequency F_(MAIN), for a dynamic receiver frequency F_(MOBILE), from a starting frequency F_(START), in the presence of high power adjacent interfering channels.

The device is connected to an antenna via a receiver and to a reference frequency F_(REFERENCE) source. The device includes controllable frequency generating means for generating an internal frequency F_(INTERNAL), frequency shift means connected to the controllable frequency generating means, and to the receiver, for shifting received frequency F_(RECEIVED), of a received channel, according to the internal frequency F_(INTERNAL).

The device also includes a frequency shift detector, connected to the frequency shift means, for detecting a frequency difference between the internal frequency F_(INTERNAL) and the received frequency F_(RECEIVED), with respect to the reference frequency F_(REFERENCE), thereby producing a frequency shift value F_(SHIFT).

The device further includes loop filtering means, connected to the frequency shift detector, for filtering the frequency shift value F_(SHIFT), thereby producing a filtered frequency shift value F_(SHIFT-FILTERED), and controlling means, connected to the controllable frequency generating means and to the loop filtering means, for determining a frequency step F_(STEP) from the filtered frequency shift value F_(SHIFT-FILTERED).

The controlling means provide the frequency shift value F_(SHIFT) to the controllable frequency generating means. The controllable frequency generating means adjust the internal frequency F_(INTERNAL) according to the frequency shift value F_(SHIFT), and the controlling means control the controllable frequency generating means to generate frequency in a range from a first frequency boundary F_(FIRST) and a second frequency boundary F_(SECOND).

According to one aspect of the invention, the controlling means set the frequency shift value F_(SHIFT) to be F_(SECOND)−F_(INTERNAL), when |F_(INTERNAL)−F_(START)|≧|F_(INTERNAL)−F_(FIRST)|, while the controlling means set the frequency shift value F_(SHIFT) to be F_(FIRST)−F_(INTERNAL), when |F_(INTERNAL)−F_(START)|≧|F_(INTERNAL)−F_(SECOND)|.

According to another aspect of the invention, the device further includes frequency filtering means, connected between the frequency shift detector and frequency shift means.

The controlling means reset the loop filtering means when setting the frequency shift value F_(SHIFT) to be F_(SECOND)−F_(INTERNAL) or F_(FIRST)−F_(INTERNAL).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

FIG. 1A is a schematic illustration of frequency versus power, describing the initial stage of a conventional initial frequency synchronization procedure;

FIG. 1B is a schematic illustration of frequency power, describing the final stage of the initial frequency synchronization procedure of FIG. 1A;

FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention;

FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention;

FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention;

FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention;

FIG. 2E is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention;

FIG. 2F is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention;

FIG. 3 is a schematic illustration of a device for synchronizing frequencies, constructed and operative in accordance with another preferred embodiment of the invention;

FIG. 4 is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with a further embodiment of the invention;

FIG. 5A is a schematic illustration of a method for operating the device of FIG. 3, operative in accordance with yet another embodiment of the invention; and

FIG. 5B is a schematic illustration in detail of a step of the method of FIG. 5A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention overcomes the disadvantages of the prior art by providing a frequency detect and fold mechanism. Accordingly, when the frequency shift exceeds a boundary value, then a predetermined frequency shift is enforced.

Reference is now made to FIGS. 2A, 2B, 2C and 2D. FIG. 2A is a schematic illustration of frequency versus power, describing the initial stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2B is a schematic illustration of frequency versus power, describing the secondary stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2C is a schematic illustration of frequency versus power, describing the third stage of a frequency synchronization procedure, operative in accordance with the present invention. FIG. 2D is a schematic illustration of frequency versus power, describing the final stage of a frequency synchronization procedure, operative in accordance with the present invention.

The schematic illustration provided by FIG. 2A describes the frequency 100 of a base station, having a value F_(BASE), a frequency 104 of a mobile unit, having an initial value F⁰ _(MOBILE) and a frequency 102 of a neighbor transmitter, having the value of F_(NEIGHBOR), wherein

 F_(BASE)<F⁰ _(MOBILE)<F_(NEIGHBOR).

In conventional communication standards, such as AMPS, NAMPS, JTACS, NTACS, USDC-TDMA and the like, the initial value of F⁰ _(MOBILE) of the mobile unit frequency 104 can be shifted from the value F_(BASE) of the base station frequency 100, by no more than a predetermined frequency gap ΔF. Another condition set by these standards is that any neighbor transmitter will transmit in a frequency F_(NEIGHBOR), which is considerably shifted from F_(BASE). Accordingly |F_(BASE)−F_(NEIGHBOR)|>2ΔF.

The method of the present invention generally searches the received spectrum within a frequency range of [F⁰ _(MOBILE)−ΔF, F⁰ _(MOBILE)+ΔF], for stabilized frequency values.

According to the invention, at the initial stage (i.e., at frequency F⁰ _(MOBILE)) the mobile unit detects all of the signals of transmitters in its vicinity and detects the frequency of the signal with the highest received power, which in the present example is the neighbor transmitted frequency 102. Accordingly, the mobile unit commences shifting its frequency 104 from the value of F⁰ _(MOBILE), towards the value F_(NEIGHBOR) of neighbor transmitter frequency 102.

The present invention makes use of the above limitations, of conventional communication standards, which outline that the initial value F⁰ _(MOBILE) of the mobile unit frequency 104 has to be within a frequency gap of ΔF from the value F_(BASE), of the base transmitter frequency 100.

Accordingly, any shift from the initial stage F⁰ _(MOBILE), cannot exceed the value of ΔF. After the frequency 104 of the mobile unit has progressed towards the neighbor transmitter frequency 102 value F_(NEIGHBOR), by a frequency shift 110, having a value of ΔF, to the value F¹ _(MOBILE), then, according to the invention, any further progress in this direction would result in a detection error and hence, should not be pursued.

At this stage, the present invention determines a reversed path 112 for frequency 104 (FIG. 2C) for shifting frequency 104 from the value of F¹ _(MOBILE) to the value of F² _(MOBILE) wherein the shift value of this reverse path 112, is a frequency gap which is twice the value of ΔF.

At the final stage (FIG. 2D) the spectrum is searched, thereby detecting the base frequency 100 as the dominant signal. Accordingly, the mobile unit 104 commences shifting its frequency towards base frequency 100, from the value of F² _(MOBILE) to F_(BASE). This shift is shown by path 114. According to the present example, no direction is enforced for path 114.

It will be noted that applying a filter, such as filter 106, improves the performance of an initial synchronization process, according to the invention. As illustrated in FIG. 2C, as long as the filter size is less than |F_(BASE)−F_(NEIGHBOR)|×2, (provided that the filter is generally symmetrical), wherein F_(NEIGHBOR) is not a high power signal, then, F_(NEIGHBOR) would not be detected as a major signal by the receiver of the mobile unit, in the original direction of progress.

Reference is now made to FIG. 3 which is a schematic illustration of a device for synchronizing frequencies, generally referenced 200, constructed and operative in accordance with another preferred embodiment of the invention.

Device 200 includes a frequency shift unit 202, an inter-mediate frequency (I.F.) filter 204 connected to the frequency shift unit 202, a frequency shift detector 206 connected to the I.F. filter 204, a loop filter 208 connected to the frequency shift detector 206, a non-linear controller 210 connected to the loop filter 208, and a voltage control oscillator (VCO) 212, connected to the non-linear controller 210 and to the frequency shift unit 202. It will be noted that VCO 212 can be replaced with any type of controlled oscillator.

The frequency shift unit 202 is further connected to an antenna 220. The frequency shift detector 206 is further connected to a host 222. The host 222 provides a reference frequency value to the frequency shift detector 206.

The antenna 220 detects frequency signals of neighbor transmitters wherein one of these detected frequency signals is transmitted by a base station. The antenna 220 provides these received frequency signals to the frequency shift unit 202. The VCO 212 generates a signal having a frequency and provides it to frequency shift unit 202.

Frequency shift unit 202 shifts frequencies, received from antenna 220, according to the frequency provided by the VCO and provides the results to the I.F. filter 204. The I.F. filter 204 filters some of these frequencies and provides the remaining ones to the frequency shift detector 206. The frequency shift detector 206 attempts to detect the frequency shift of each of these shifted frequencies from the reference frequency value, provided by the host 222.

Accordingly, the frequency shift detector 206 determines a frequency shift value and provides it to the loop filter 208. The loop filter 208 includes the history of the frequency shifts performed by device 200 and accordingly determines a frequency shift direction and provides it with the frequency shift value to the non-linear controller 210.

The non-linear controller 210 detects if the overall shift, up until this stage has exceeded the value of ΔF. If so, then the non-linear controller 210 provides VCO 212 with the command to generate a reversed frequency shift such as the one according to path 112 (FIG. 2C). If not, then the non-linear control 210 provides the VCO 212 with a frequency shift value and a frequency shift direction for further shifting the frequency towards the most dominant received frequency. Then the VCO 212 provides a new shift frequency to the frequency shift unit 202 and the process is repeated from the beginning.

It will be noted that when using a slow loop filter, such as software implemented loop filter, it would be difficult for such a loop filter to process a considerable shift such as the one defined by path 112, since such shifts are compared to frequency behavior history contained therein.

According to a further aspect of the invention, when the non-linear controller 210 determines a 2ΔF shift, it also sends a clear command back to the loop filter 208, thereby erasing the frequency history contained in the memory of loop filter 208. This operation enables the loop filter 208 to further process considerable frequency shifts.

It will be noted that the terms base, mobile and neighbor are presented as a matter of convenience only. The present invention is applicable for any type of initial frequency acquisition in the presence of a high power adjacent channels, wherein the base of the above example is assigned to a main transmitter, the mobile of the above example is assigned to a receiver and the neighbor of the above example is assigned to an adjacent interfering transmitter.

It will be noted that each of the main transmitter, the adjacent transmitter and the receiver may be implemented for a mobile unit, a base unit and the like.

Reference is now made to FIG. 4 which is a schematic illustration of a method for operating the device 200 of FIG. 3, operative in accordance with a further embodiment of the invention.

In step 300, the device 200 stores the value F⁰ of the internal initial frequency F. F⁰ is used to determine, later on, the total amount of shift from the initial frequency. It will be noted that for this purpose, the device 200 can store and accumulate the values of the later frequency shifts, instead.

In step 302, the device 200 detects incoming frequency signals.

In step 304, the device 200 filters the incoming frequency signals, thereby obtaining selected frequencies.

In step 306, the device 200 determines a target frequency value F_(TARGET), from the selected frequencies. In the present example (FIG. 2A), the device 200 (FIG. 3) selects the right side signal 102 (F_(NEIGHBOR)), as the target frequency F_(TARGET).

In step 308, the device 200 progresses the internal frequency F towards the target frequency F_(TARGET) by a predetermined frequency step F_(STEP). It will be noted that F_(STEP) can be determined using a range of considerations, such as speed, accuracy and the like. In general, F_(STEP) is determined to be significantly smaller than ΔF, thereby yielding higher accuracy. It will further be noted that F_(STEP) can be infinitesimal thereby yielding an analog like behavior.

In step 310, the device 200 detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device 200 proceeds to step 312. Otherwise, the device 200 proceeds to step 314.

In step 312, the device 200 reverses F by 2ΔF. In the present example (FIG. 2C), reverse path 112, describes such a reverse shift, from the value of F¹ _(MOBILE) to the value of F² _(MOBILE). Then, the device 200 repeats the steps of the above method, from step 302.

It will be noted that at this stage, signal 102 appears to be outside of the filtering bandwidth of filter 106, thereby leaving the base station frequency signal 100, the strongest, at the output of filter 106. Accordingly, the device 200 determines F_(BASE) as F_(TARGET).

In step 314, the device 200 detects if the internal frequency F is synchronized with the target frequency F_(TARGET). If so, then the device 200 has completed the initial frequency acquisition procedure and accordingly, locks the frequency F (step 316). Otherwise, the device 200 repeats the steps of the above method, from step 302.

The method of FIG. 4 overcomes a situation where there exists interfering neighbor frequencies such as F_(NEIGHBOR) (reference numeral 102) on one side of the spectrum.

In a situation where there exist interfering neighbor frequencies on both sides of the base frequency F_(BASE), the present invention provides a slightly different solution, as will be disclosed hereinbelow.

Reference is now made to FIGS. 2E and 2F. FIG. 2E is a schematic illustration of frequency versus power, describing a stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention. FIG. 2F is a schematic illustration of frequency versus power, describing a final stage of a frequency synchronization procedure, operative in accordance with another aspect of the present invention.

According to the present example, there exists an additional neighbor frequency 120 having a value of F*_(NEIGHBOR), on the left side of the base frequency 100 F_(BASE). When the mobile frequency completes the 2ΔF frequency shift 112, additional neighbor frequency 120 falls within the filtering bandwidth of filter 106, together with base frequency 100.

It will be noted that if, at the output of filter 106, the signal of the additional neighbor frequency 120 appears to be stronger than the signal of the base frequency 100, then, according to the method of FIG. 3, the mobile frequency 104 would be drawn towards the additional neighbor frequency 120.

According to another aspect of the present invention, the initial direction set forth in the second stage (i.e., the direction of frequency shift 110, (FIG. 2B)), is stored. In the present example, this direction is from left to right.

Then, after the mobile frequency completes the 2ΔF frequency shift 112, the acquisition mechanism continues searching in that initial direction, only. It will be noted that such forced search direction provides an accurate acquisition of the desired base frequency, in one or less search cycle.

In a more detailed form, at the final stage (FIG. 2F) the spectrum is searched again in the direction set forth in the initial stage (i.e., the direction of shift 110), thereby detecting the base frequency 100 as the dominant signal. Accordingly, a path 122 is set towards base frequency 100, for shifting mobile frequency 104 from the value of F² _(MOBILE) to F_(BASE).

It will be noted that the present invention provides a search shift step which can be calibrated at each search stage. For example, on the one hand, in the presence of a powerful additional neighbor 120, frequency shift 122 may include a large number of infinitesimal frequency shift steps. Otherwise, frequency shift 122 may include a small number of larger frequency shift steps.

Reference is now made to FIGS. 5A and 5B. FIG. 5A is a schematic illustration of a method for operating the device 200 of FIG. 3, operative in accordance with yet another embodiment of the invention. FIG. 5B is a schematic illustration in detail of step 406 of the method of FIG. 5A.

In step 400, the device 200 stores the value F⁰ of the internal initial frequency F.

In step 402, the device 200 detects incoming frequency signals.

In step 404, the device 200 filters the incoming frequency signals, thereby obtaining selected frequencies.

In step 406, the device 200 determines frequency step F_(STEP) and a frequency advance direction, in a way which is described in detail in FIG. 5B.

In step 418, if the detection performed according to step 402 is the first detection in the current acquisition cycle, then the device 200 proceeds to step 420. Otherwise, the device 200 proceeds to step 408.

In step 420, the device 200 determines an initial advance direction which will be constant during the present acquisition cycle, and proceeds to step 408.

In step 408, the device 200 progresses the internal frequency F by frequency step F_(STEP), in the advance direction.

In step 410, the device 200 detects if the internal frequency F was shifted beyond a gap of ΔF. If so, then the device 200 proceeds to step 412. Otherwise, the device 200 proceeds to step 414.

In step 412, the device 200 reverses F by 2ΔF. In the present example (FIG. 2E), reverse path 112, describes such a reverse shift, from the value of F¹ _(MOBILE) to F² _(MOBILE). Then, the device 200 repeats the steps of the above method, from step 402.

It will be noted that at this stage, additional neighbor frequency signal 120 falls within the filtering bandwidth of filter 106, which poses a problem if additional neighbor frequency signal 120 appears stronger than the base station signal 100, at the output of filter 106.

Referring now to FIG. 5B, the device 200 determines a target frequency value F_(TARGET) from the selected frequencies (step 430). In the present example, when the mobile frequency is at a value of F⁰ _(MOBILE) (FIG. 2A), the device 200 (FIG. 3) selects the right side signal 102 (F_(NEIGHBOR)), as the target frequency F_(TARGET). Alternatively, when the mobile frequency is at a value of F² _(MOBILE) (FIG. 2E), the device 200 (FIG. 3) selects the left side signal 120 (F*_(NEIGHBOR)), as the target frequency F_(TARGET).

In step 432, if the detection performed according to step 402 is the first detection in the current acquisition cycle, then, the device 200 proceeds to step 440. Otherwise, the device 200 proceeds to step 434.

In step 434, the device 200 determines an advance direction from the mobile frequency value F and the target frequency value F_(TARGET).

In step 436, if the advance direction determined in step 434 is equal to the initial advance direction, determined in step 420, then the device 200 proceeds to step 440. Otherwise, the device 200 proceeds to step 438. It will be noted that a situation where these directions are not equal occurs, for example, when a neighbor signal, such as the one of additional neighbor frequency 120, appears to be stronger than the signal of the base frequency 100, at the output of the filter 106.

In step 440, the device 200 determines the frequency step F_(STEP) according to the position of F and F_(TARGET). In the present example, F_(STEP)≦|F−F_(TARGET)|.

In step 438, the device 200 determines the advance direction to be the initial advance direction.

In step 442, the device 200 determines the frequency step F_(STEP) relatively small. It will be noted that, according to the present example, the size of F_(STEP) is smaller, compared to the size of ΔF.

Referring back to FIG. 5A, wherein if the device 200 detects if the internal frequency F is synchronized with the target frequency F_(TARGET) (step 414), then the device 200 proceeds to step 416 and locks F. Otherwise, the device 200 repeats the steps of the above method, from step 402.

Hence, the method of FIGS. 5A and 5B overcomes a situation where there exist interfering neighbor frequencies such as F_(NEIGHBOR) (reference numeral 102) and F*_(NEIGHBOR) (reference numeral 120) on either side of the F_(BASE).

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the claims which follow. 

What is claimed is:
 1. An apparatus comprising: a frequency shift detector to detect a frequency difference between an internal frequency and a received frequency with respect to a reference frequency and to produce a frequency shift value; a non-linear controller to control an oscillator to generate said internal frequency in a range from a first frequency boundary to a second frequency boundary, and to provide said frequency shift value to said oscillator so that said oscillator adjusts said internal frequency according to said value; and a loop filter coupled to said frequency shift detector to filter said frequency shift value and to store information regarding previous frequency shifts and accordingly to determine a frequency shift direction.
 2. The apparatus of claim 1, wherein said controller is able to determine a frequency step.
 3. The apparatus of claim 1 further comprising: a frequency shift unit coupled to said oscillator.
 4. The apparatus of claim 1, wherein said controller generates a command to shift said internal frequency to said second boundary value when said internal frequency reaches said fist boundary value.
 5. The apparatus of claim 1, wherein said controller generates a command to erase the information regarding previous frequency shifts stored in said loop filter.
 6. The apparatus of claim 1, wherein said controller generates a command to shift said internal frequency to said first boundary value when said internal frequency reaches said second boundary value.
 7. The apparatus of claim 6, wherein said controller further generates a command to erase the information regarding previous frequency shifts stored in said loop filter.
 8. A method comprising: progressing a dynamic receiver frequency in an advance direction towards a carrier frequency of a dominant signal detected within a filtering bandwidth by no more than a predetermined frequency gap; and shifting said dynamic receiver frequency in the opposite direction to said advance direction by twice said gap once said dynamic receiver frequency has progressed from an initial frequency in said advance direction by said predetermined frequency gap.
 9. The method of claim 8 further comprising: progressing said dynamic receiver frequency only in said advance direction after shifting said dynamic receiver frequency in said opposite direction.
 10. The method of claim 8, wherein progressing is performed in a frequency step.
 11. The method of claim 10, wherein the value of said frequency step is infinitesimal in comparison to said predetermined frequency gap.
 12. The method of claim 10 further comprising adjusting said frequency step.
 13. The method of claim 8, wherein said initial frequency is shifted from the carrier frequency of the desired signal by not more than said predetermined frequency gap.
 14. The method of claim 8 further comprising: progressing said dynamic receiver frequency in said advance direction towards the carrier frequency of the desired signal. 